The invention relates to a belt tensioner and a belt tensioning system, but more particularly the invention relates to a spring type tensioner used with a camshaft belt drive system for automotive engine applications.
In an automotive camshaft drive system, a synchronous belt or timing belt is entrained around pulleys including at least two toothed pulleys. One of the pulleys is a crank pulley and the other is one or more camshaft pulleys that induce cyclic torque variations into the drive system. An idler pulley such as a back-side idler pulley is pressed against the belt to effect an installed belt tension. It is common practice in many timing belt drive systems to compromise the amplitude of belt tension variations in a system by means of a fixed idler pulley. An installation belt tension at room temperature is chosen to inhibit the amplitude of the belt tension variations to prevent belt toothed failure and avoid tooth jump (i.e., ratcheting). If the installed belt tension is too low, belt tooth failure and tooth jump can occur. If installation tension is too high, it can introduce shortened belt life and belt noise at engine operating temperatures. A fixed idler pulley must operate over a range of temperature conditions. There is a change in pulley center distance between a cold automotive engine such as usually occurs during engine start up and a warm or thermally expanded engine such as occurs during normal engine operating temperatures. Thus, the thermal effect is to increase belt tension with engine temperature increases and conversely, decrease belt tension with decreases in engine temperature.
It is known to use an automatic tensioner in conjunction with a synchronous or timing belt drive system to compensate or handle tension variations in a belt that are attributable to dynamic effects such as cyclic torque variations or thermal effects such as those that introduce a change in the length of a timing belt drive. The tensioner is located at a normally slack side belt span of a belt in a belt drive system. An example of such spring type, mechanical tensioners for a synchronous belt drive are disclosed in U.S. Pat. Nos. 5,045,031 and 5,098,347. While these tensioners are designed to accommodate cyclic torque variations and thermal effects in a belt drive system by controlling belt tension at a slack side belt span, such tensioners are not designed to accommodate extreme torque reversal situations such as those that can occur during engine backfiring or when an engine is rotated in reverse (e.g. an automobile going backward while in forward gear with the clutch out). In such extreme torque reversal situations, the slack side of the belt drive system becomes the tight side of the belt and vice versa. Tight side belt tension at the belt tensioner causes the tensioner to operate to "kickback" to rapidly decrease belt tension by moving a pulley and its pivot-arm rapidly away from the belt to slacken the tight side belt span. If the pulley movement is extreme, it can over slacken the belt which may result in tooth jump or ratcheting as the slackened belt enters the crank pulley or cam shaft pulleys. Tooth jump or ratcheting is deleterious to the operation of an engine as synchronization of the pulleys is lost.
Some tensioners have a ratchet and pawl mechanism attached to the tensioner's pivot arm to eliminate tensioner "kickback" and avoid tooth jump or ratcheting. U.S. Pat. No. 4,299,584 discloses a ratchet operative with a leaf-spring pawl that allows some compliance at kickback by permitting the leaf-spring to slightly deflect. Another ratchet and pawl mechanism is disclosed in U.S. Pat. No. 4,634,407 where the ratchet mechanism operates as a one-way clutch that fixes the position of a pivot-arm so that the tensioner cannot operate to slacken a belt. A problem with such ratchet/pawl devices is that the tensioner must operate as substantially a fixed idler in one direction as the ratchet mechanism limits the motion of the tensioner pivot-arm. In other words, the tensioner pivot-arm cannot operate in a direction that would allow the belt to be slackened. The problem of belt tooth failure and noise is reintroduced into the belt drive system when a belt cannot be slightly slackened.